Giant Rabi splitting in a bulk CuCl microcavity

We have investigated exciton-photon coupling phenomena in a bulk CuCl microcavity with distributed Bragg reflectors consisting of ${\text{PbF}}_2$ and ${\text{PbBr}}_2$ layers prepared by vacuum deposition. Reflectance spectra observed at various incident-light angles demonstrate the strong-coupling behavior of the exciton and photon modes; namely, the energies of reflectance dips exhibit incident-angle dependence accompanied by an anticrossing behavior peculiar to the cavity polaritons. In addition, the emission from the lower polariton branch was detected with angle-resolved photoluminescence spectroscopy. From the phenomenological analysis with a $3\times 3$ Hamiltonian of the cavity-polariton modes originating from the $Z_3$ exciton, $Z_{1,2}$ exciton, and photon, it has been revealed that the vacuum-Rabi-splitting energies are 97 and 162 meV for the $Z_3$ and $Z_{1,2}$ excitons, respectively. These giant Rabi-splitting energies result from the large oscillator strengths of the relevant excitons. Furthermore, the spectral profile of reflectance is reasonably explained by calculations based on a theory of nonlocal linear optical response.

Figure 1

(Color online) Refractive index profile of the CuCl microcavity structure together with the calculated result of the electric-field intensity $\left({\left|E\right|}^2\right)$ along the growth direction for normal incidence at a photon energy of 3.202 eV that is resonant with the $Z_3$-exciton energy of CuCl.

Figure 2

(Color online) Angle-resolved reflectance spectra from the CuCl microcavity at 10 K. The dip positions labeled with open circles correspond to the strongly coupled cavity polariton, and those labeled open squares are due to weakly coupled modes. The dashed vertical lines indicate the energy positions of the $Z_3$ and $Z_{1,2}$ excitons of CuCl. The dotted curves are a guide for the eyes.

Figure 3

(Color online) Angle-resolved PL spectra at 10 K with the corresponding reflectance spectra. The down arrows indicate the peak energy of the PL band due to the LPB. The PL band labeled $I_1$ is attributed to the bound exciton. The PL band labeled 2LO is the two-LO-phonon replica of the weakly coupled modes in the energy region slightly higher than the $Z_3$-exciton energy.

Figure 4

Angle dependences of the energy positions of three major dips (open circles) and four minor dips (open squares) in the reflectance spectra. The solid curves are the incidence-angle dependence of the energies of the strongly coupled polariton modes calculated using the $3\times 3$ Hamiltonian. The dashed lines represent the energies of the bare $Z_3$ exciton, $Z_{1,2}$ exciton, and cavity photon.

Figure 5

Calculated reflectance spectra based on the nonlocal theory of linear optical response. The vertical dashed lines indicate the energies of the $Z_3$ and $Z_{1,2}$ excitons. The fine structures originating from the weakly coupled modes appear in the shaded areas in addition to the strongly coupled modes.